Lyle, have you ever considered keeping the shoulders down at the top of the rep as a way to keep the lats better engaged? Since they function to depress the shoulders as well, I find simply preventing the big shoulder shrug (they do come up a bit) during the negative gets them working more consistently. It almost seems like a rest for them at the top, IMHO.

No, I don't think it's normal shoulder function and depressing the shoulder is one of their actions, I think it should be trained. And since I've never had trouble teaching probably shoudler movement with pulldowns or rows, I see no reason not to teach it.